In a sophisticated spear phishing campaign, French-speaking organizations are receiving bogus calls asking them to check an invoice, which is actually malware

Hackers are finding it pays to call ahead before sending malware-laden email.

Symantec has seen what it describes as a sophisticated social engineering campaign aimed at French-speaking accounting and finance department employees. The victim is called and asked in French if they can process an invoice sent by email.

The style of attack, known as "spear phishing," has been used against French organizations, including subsidiaries in Romania and Luxembourg.

"There is evidence to suggest that these attacks began as early as February 2013, however, it was only more recently in April that phone calls were being placed prior to sending the victim the phishing email," Symantec wrote on its blog.

The attackers obtain the victim's phone number and email -- both pieces of information that are generally easy to find. Their targets would have access to sensitive financial information, and handling invoices would be part of their normal course of business.

"Since handling invoices is something they would do on a regular basis, this lure has the potential to be quite convincing," Symantec wrote. "Each element of this attack requires careful planning and contributes to the overall success rate of the attack."

The email contains either a malicious link or an attachment, which is malware. Symantec said the attached program is a variant of "W32.Shadesrat," which is a remote access Trojan that hackers use to steal information from a computer.

Shadesrat can steal passwords and conduct distributed denial-of-service attacks, according to a writeup from Symantec from 2011.

Shadesrat can be licensed for $40 to $100 a year, Symantec said.

The company said the attackers may have just limited information on their targets and recommended those receiving a call ask additional questions to verify the caller is legitimate. Sensitive information should also be encrypted.

"Organizations also need to be aware that personally identifiable employee information that exists outside of your enterprise, even in the form of an invoice, can be used against you if a business associate becomes compromised," Symantec wrote.